The present invention relates to a hydraulic control system for an automatic transmission.
Automatic transmissions are known wherein one friction unit is engaged and another friction unit is disengaged to effect a shifting between speed ratios, i.e., between drive ratios. In operating such automatic transmissions, if the engagement of the one friction unit does not take place in good timed relationship with the disengagement of another friction unit, substantial shocks will take place or an engine will be raced. Particularly, a shifting which is to be effected by engaging one multiple plate clutch and disengaging another multiple plate clutch is difficult and substantial shocks are unavoidable because it is very difficult to controllably engage or disengage the clutch at a gradual rate. The substantial shocks take place particularly when a shifting is made from a n+2th speed ratio to a nth speed ratio skipping n+1th speed ratio (where, n is a positive integer) because a difference between an engine revolution speed before the shifting and that after the shifting and a difference between a driving torque before and that after the shifting are big.